Seismic waves generated artificially have been used for more than 50 years to perform imaging of geological layers. During seismic exploration operations, vibrator equipment or dynamite (also known as a “source”) generates a seismic signal that propagates in the form of a wave that is reflected at interfaces of geological layers. For land seismic surveying, these reflected waves are typically received by geophones, or more generally “receivers”, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal which is recorded. Analysis of the arrival times and amplitudes of these waves make it possible to construct a representation of the geological layers on which the waves are reflected.
FIG. 1 depicts schematically a system 100 for transmitting and receiving seismic waves intended for seismic exploration in a land environment. The system 100 comprises a source 102 consisting of a vibrator operable to generate a seismic signal, a set of receivers 104 for receiving a seismic signal and converting it into an electrical signal and a recorder 106 for recording the electrical signals generated by the receivers. The source 102, the receivers 104 and the recorder 106 are positioned on the surface of the ground 108. FIG. 1 depicts source 102 as a single vibrator but it should be understood that the source may be composed of several vibrators, as is well known to persons skilled in the art.
In operation, source 102 is operated to generate a seismic signal. This signal propagates firstly on the surface of the ground, in the form of surface waves 110, and secondly in the subsoil, in the form of transmitted waves 112 that generate reflected waves 114 when they reach an interface 115 between two geological layers. In a solid medium, the waves radiated by a source (transmitted waves 112) are a combination of P-waves (pressure waves) and S-waves (shear waves). P-waves as they pass through the media produce localized volumetric changes in the media; while, S-waves produce a localized distortion in the media with corresponding particle motion but without any net volumetric change. The surface wave 110 produces a retrograde particle motion in the soil, but there is no local volumetric change associated with it as it propagates. The propagation velocity for surface waves and S-waves is much less than for P-waves. Typically the fraction of P-wave radiated energy from a vertical surface source is about 8%, with surface waves and S-waves comprising the remaining 92% of the total radiated wave energy. Surface waves 110, decay with depth, but they decay more slowly at low frequencies, so they can still have significant amplitude even at 100 m depth for example. Each receiver 104 receives both a surface wave 110 and a reflected wave 114 and converts them into an electrical signal, which signal thus includes a component associated with the reflected wave 114 and another component associated with the surface wave 110. Since system 100 intends to image the subsurface regions 116 and 118, including a hydrocarbon deposit 120, the component in the electrical signal associated with the surface wave 110 is undesirable and should be filtered out. In general, most reflection seismology today use the reflection data associated with P-wave emissions and their reflections. In many cases, S-waves are not used and oftentimes treated as another undesired source of coherent noise. For the case of reservoir monitoring, where a high degree of repeatability may be required, it should be noted that source 102 may be a buried source rather than a surface source. One such reservoir monitoring system that employs buried sources is described in U.S. Pat. No. 6,714,867. Buried receivers can also be useful for monitoring/imaging other oil-field processes like fracture monitoring where the receiver is located closer where a microseismic event might be created by fluid injection; or for passive seismic monitoring in which case the seismic source may be drills, natural phenomena like earthquakes or ocean tides.
Historically, land seismic systems 100 have typically employed geophones as receivers 104. A geophone is a device that converts ground movement into voltage. Geophones use either a spring mounted magnetic mass or a spring mounted coil. More recently an analogous MEMS device has been introduced. The deviation of this measured voltage from a base line is the seismic response which can be analyzed to image the subsurface regions 116, 118 and 120. By way of contrast, hydrophones have typically been employed for marine seismic systems. A hydrophone is essentially a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Such piezoelectric materials or transducers can convert a sound signal into an electrical signal since sound is a pressure wave. Although geophones have typically been used as receivers 104 in land seismic operations, and hydrophones have typically been used as receivers in marine seismic operations, in certain cases these roles have been reversed and indeed today some seismic systems are being designed to use both types of sensors as receivers.
For example, although receivers 104 typically sit on top of the ground in land seismic systems, there is another class of receivers that can be placed in boreholes. These borehole receivers are generally known to those of skill in the art, and an example is described in U.S. Pat. No. 6,584,038 (“the '038 patent”) using a hydrophone as the fundamental receiver component. More specifically, as illustrated in FIG. 2 (which is reproduced from the '038 patent), the seismic wave reception device described therein includes a hydrophone 200, and a closed, flexible-walled sheath 202 filled with a liquid 204. The sheath 202 is closed at one end by a sealed plug 206 provided with a sealed duct for a cable 208 which connects the hydrophone 200 to a signal acquisition device (not shown). Because well-designed hydrophones are more sensitive to pressure changes than to particle motion, hydrophones have been found useful for rejecting surface waves and S-waves.
The device shown in FIG. 2 is intended to be tightly coupled with an edge 210 of the medium 212 (e.g., soil and/or rock) in which a borehole having the hydrophone 200 is created for seismic exploration. To accomplish this tight coupling between the hydrophone 200, liquid 204, sheath 202 and the medium 210, 212, a hardenable material 214 (such as cement) is filled in between these elements of the hydrophone receiver device and the edge 210 of the medium 212
While it has been found in general that hydrophones, such as that described in the '038 patent, work well in land seismic applications when they are located below the water table and are fluid coupled with a formation to be imaged, there are many other land seismic applications wherein such constraints would make hydrophones generally, and the receiver device described in the '038 patent specifically, unsuitable. For example, there are many areas that need to be monitored that have very deep water tables, such as deserts, so that it is economically impractical to bury hydrophones at great depth.
Another problem inherent to such a device is that if it is positioned in an area of rock, or some other hardened medium, the use of liquid 204 around the hydrophone creates a mismatch in acoustoelastic properties, much like a cable impedance mismatch in the transmission of electromagnetic signals via cables (especially at microwave frequency signals). Accordingly, some of the pressure information which would otherwise be generated by the seismic waves would be lost in the transfer between the different media. Yet another potential difficulty associated with such a device is that if the liquid 204 leaks out of the device illustrated in FIG. 2, it would become almost useless for its intended purpose of measuring received seismic waves.
Accordingly, it would be desirable to provide methods, modes and systems for a buried land seismic receiver that overcomes at least one or more of these problems.